The invention relates to the inspection of printed circuit boards and, more particularly, to methods and apparatus for imaging component leads in assembled printed circuit boards. The invention has application in the testing of printed circuit boards prior to wave soldering.
To reduce production costs, and to improve quality and reliability, a printed circuit board is checked at many stages throughout its manufacture. One critical testing stage, for example, is in the period following assembly of electrical components on the board and prior to soldering of the components' leads. Detection of faults at this stage facilitates the inexpensive repair of assembly defects, which may be impossible to detect after soldering.
In the pre-solder stage, the assembled boards are difficult to inspect. They cannot be inspected electrically because reliable connections are not yet established. The testing method of choice therefore is visual inspection, preferably by machine. This inspection is intended to reveal faulty component lead placement, including improper lead bending, which may prevent a component lead from passing through a component hole, and improper lead direction, which may result in a lead contacting a neighboring solder pad.
Visual inspection of printed circuit boards by machine requires the use of a device for producing images representative of component lead placement. Among the prior art methods for producing such images is the so-called "bright leads technique." According to this process, an assembled board is illuminated from the side. A camera is placed above the board and, specifically, above a component hole to be inspected. If a component lead has been properly inserted through the hole, light from the illuminating source reflects off the lead and toward the camera. This reflection appears as a bright spot centered about the component hole. In the event a component lead has not been properly inserted, no bright spot appears at the camera.
One drawback of the bright leads technique is that it does not consistently produce a bright spot, even when the component lead is properly positioned. This defect arises, in part, because component leads are highly specular surfaces, i.e., they reflect incident light in a very small cone. If a properly placed lead is not oriented in the expected direction, reflection from the lead will not reach the inspection camera lens. Consequently, a bright spot will not appear in the image, and a defect will be indicated where none actually exists.
The art provides other imaging devices employing variations on the basic bright leads technique which are intended to reduce problems caused by lead misorientation. These devices use multiple cameras to catch light reflected in a number of directions. Although this technique increases the likelihood that at least one camera will see the lead, it still provides no guarantee that a properly placed lead will actually be detected. Moreover, the use of multiple cameras increases both the cost of the testing equipment and the time required to examine the images produced by those cameras.
Kurtz et al, U.S. Pat. No. 4,240,750, discloses another prior art technique for imaging component lead placement. According to this publication, a laser beam is scanned, for example, on a degree-by-degree basis around the solder pad of a printed circuit component hole. Presence of a lead on the solder pad is signified by differing beam reflectance at the spot where the lead crosses the solder pad. One drawback of the Kurtz et al technique is the expense required to practice it. Another is the enormous amount of time required to inspect each solder pad while the laser beam makes its step-by-step procession around the pad parameter.
An object of this invention is to provide improved imaging equipment for use in the inspection of assembled printed circuit boards. More particularly, an object is to provide an improved method and apparatus for generating images of component lead placement in circuit board component holes.
Another object of the invention is to provide a system for printed circuit board component lead imaging which utilizes relatively inexpensive equipment.
Still another object of the invention is to provide a reliable system for printed circuit board component lead imaging which produces images incorporating necessary information to facilitate interpretation of lead placement.
Yet another object of the invention is to provide a system for printed circuit board component lead imaging which may be readily adapted to pre-existing assembly line equipment.
Still further objects of the invention are evident in the drawings and description which follows.